Electron discharge device



Sept 0, 1957 w. P. BENNETT ETAL 2,306,166

ELECTRON DISCHARGE DEVICE Filed Aug. 50, 1952 United States Patent ELECTRON DISCHARGE DEVICE Wilfred P. Bennett and Henry F. Kazanowski, Lancaster,

Pa., assignors to Radio Corporation of America, a corporation of Delaware Application August 30, 1952, Serial No. 307,318

11 Claims. (Cl. 313-174) This invention relates to electron tubes, and particularly to electron tubes useful at very high and ultrahigh frequencies.

The provision of a tube which will operate at ultrahigh frequencies and deliver large amounts of continuous power with reasonable band width and power gains presents serious problems incapable of solution with conventional tube designs.

The dissipation of heat, which is one of the most serious problems and which must be accomplished to protect seals and electrodes and to prevent undesired electron emission, requires structures which, if conventional designs for low frequency tubes were used, would adversely affect the electrical characteristics of the device in the way of increased inductances and capacitances and increased electron transit times. This limits operation at high frequencies, particularly when the desired band width is that required for television. Heat causes expansion and contraction of the tube elements, thus affecting spacings and alignments and operating stability. The need to dissipate heat from all electrodes and elements introduces further cooling problems since for high frequency operation small structures and close spacings are a prerequisite.

High frequency operation on the other hand requires small electrodes, short leads and close spacing to reduce interelectrode and lead capacitance and inductance and to limit the transit time of the electrons between the electrodes to such value that the transit time does not become an appreciable percentage of the period of oscillation. That is, the transit angle must be small. However, close spacing increases interelectrode capacitances and consequently the electrode area must be decreased and thus for high power output, cooling becomes difficult with such compact structures.

At high frequencies stable and efficient operation can be had only by properly isolating the input and output circuits and electrodes.

Also, since tubes operated at very high or ultra high frequencies are usually used in conjunction with cylindrical resonators, it is desirable that the electrode terminals be made in the form of concentric ring contacts. Coaxial electrode assemblies are likewise desirable from the standpoint of symmetrical energy distribution within the tube.

In addition, if the tube is to have a long uesful life the cathode of the tube must have sufficient area so that the required electron emission does not exceed a critical amount per unit area of emissive surface.

Also, if the tube is to maintain a high vacuum over a long period of time, it is necessary that some means he provided for gettering the tube.

The importance of the above design considerations has been recognized for several years, and tubes constructed in accordance therewith and having a low power output have been made for a considerable time.

One such tube which is commercially available is disclosed and claimed in the co-pending application of L.

ice

P. Garner et a1., Serial No. 26,696, filed May 12, 1948, and entitled Electron Tube Construction and since refiled on February 26, 1953, Serial No. 339,002." This low power tube has coaxial and concentric close-spaced electrodes and coaxial ring electrode terminals so that the tube may easily be used in conjunction with external concentric resonators.

In view of the fact low power tubes capable of ultra high frequency operation have been made for some time, it might be expected that tubes capable of outputs of the order of a kilowatt or more at ultra high frequencies and having a long useful life could be made by scaling up the dimensions of the low power tubes while maintaining the required close intereelctrode spacing which is necessary if the transit angle is to remain small. Such scaled-up tubes would not prove entirely satisfactory, because increasing the length of the cathode emitting surface would limit the upper operating frequency of the tubes. The upper operating frequency would be affected by the increased inductance and capacitance and by the length of the cathode becoming an appreciable fraction of a wavelength at the upper operating frequency. Shortening the length of the cathode would correct this deficiency, but at the same time reduce the area of the emitting surface. This would require that the electron emission density per unit area of emitting surface be substantially increased, if the power output were to remain at the desired high level. Thus the tube life would be shortened because of the excessively high electron emission from the shortened cathode.

The cathode emitting surface could be increased by increasing the diameter of the cathode while restricting length to a suitable dimension. However other problems arise when an enlarged diameter cathode is incorporated in an ultra high frequency tube.

One difficulty is that a conventional coil heater is unsuited to a large diameter cathode since it would require a supporting structure to overcome its tendencies to sag while heating and thus cause a short circuit between the heater and cathode.

Furthermore, in order to prevent undesirable modulation of the tubes output, the heater winding field should not extend into the active area of the cathode-grid-anode area of the tube. Shielding of the heater field from the active area of the tube is usually achieved by closing the top of the cathode, but in a large diameter cathode this would increase the volume to be heated, requiring more heater power and also increase interelectrode capacity. Also, the top of the cathode might prove an annoying source of undesired electron emission.

Thermal expansion causes another problem in tubes having a cathode of large diameter which doesn't usually occur in low power tubes. Thermal expansion, for a given material, is expressed in units of expansion per unit of length per degree of temperature change. Electrodes in tubes having cathodes of enlarged diameter are thus subject to perhaps severals times as much actual thermal expansion of their diameters as are the small diameter electrodes of the lower power tubes. Yet, because of transit time considerations, interelectrode spacings between electrodes of the high power tubes must be held to the close interelectrode spacings of the lower power tubes. This requirement calls for electrodes having uniform expansion characteristics relative to each other and relative to different portions of the same electrode. It is perhaps equally important that warping of the part does not occur and that, on cooling, the part will return to the position it had before it was heated. Otherwise, cumulative warping effects might cause undesirable changes in the performance characteristics of the tube.

In addition, the high temperatures required to make ceramic-to-metal seals or metal-to-metal seals during assembly of the tube may also cause mechanical distortions, such as warping and oxidation of metal parts, which adversely affect operation of the tube. The high temperature effects, such as warping, make difficult the problem of providing jigs which maintain the parts in accurately spaced relationship during assembly of the tube.

Gettering an ultra high frequency power tube presents yet another problem. The getter materials should not be deposited on active electrode portions of the tube when the getter is flashed. Neither should the flashed material provide a short circuit conductive path between adjacent parts of different potential. Also, the getter structure should be so disposed that it does not interfere with the symmetry of radio frequency voltage distribution within the tube. Furthermore, it is desirable that the getter may be fired independently of the voltages which are applied to the other electrodes of the tube.

A principal object of the present invention is to provide an improved electron tube capable of large power output at ultra high frequencies.

Another object of the present invention is to provide an improved electron tube capable of an output of the order of a kilowatt or more at ultra high frequencies and having a long useful life.

An additional object of the present invention is to provide an improved cathode assembly of high thermal efficiency and in which the heater field is isolated from the active portion of the tube.

Still another object of the present invention is to provide an electron tube having an improved indirectly heated cathode of enlarged diameter and which has a long useful life.

A further object of the present invention is to provide an air cooled electron discharge device having improved non-deforming electrode and envelope structures.

A still further object of the present invention is to provide, in a power electron tube, strain-relieving, nondeforming grid structures.

A more specific object of the present invention is to provide an improved means for gettering an electron tube.

An ultra high frequency tube made in accordance with the present invention has an indirectly heated cylindrical offset cathode structure, the active or emitting portion of the cathode being the peripheral surface of the offset portion. The grid structures are nested around the cathode and have cylindrical grid supports which terminate in ring contacts which are a part of the tube envelope. The grids and cathode are telescoped within an inverted cup-shaped anode which is also a part of the tube envelope. The several metal envelope portions are insulated one from another by ring-like ceramic envelope portions. The tube anode has a plurality of fins to facilitate air cooling of that electrode.

The improved cathode assembly of the present invention comprises a cathode and a tubular supporting structure. The cathode is a hollow ring-like cylindrical element of U-shaped cross section, the base of the U facing outwardly, and is conductively secured to one end of a tubular support. The cathode is energized by a toroidally wound insulated heater which is disposed within the U-shaped recess of the hollow cathode element.

To prevent large heat loss and to provide cathode heater field shielding a cup-shaped heat shield having an outwardly flanged open edge is secured around its flanged edge to the top of the cathode, with the cup part of the shield extending downwardly through the center of the ring-like cathode element and into the tubular cathode support.

The envelope portions of the tube are hermetically sealed one to another by tapered self-aligning ceramicto-metal seals which obviate the need for additional jigging of the tube envelope sections during firing. The parts are stacked and then inserted in an oven Where the sealing is done under non-oxidizing atmospheric conditions. Because all parts are heated substantially to the same temperature, there is a minimum of warping of the tube envelope or the internal structures due to uneven temperature distribution.

In further accordance with the present invention, a rotatable top cap is provided for the grid structures in order to maintain equal thermal expansion between the upper and lower portions of the grid. The grid top cap is supported from the upper periphery of the grid by a plurality of similarly arranged arms disposed non-perpendicularly with respect to a tangent to the periphery. The angular arrangement of the arms causes the motion due to thermal expansion of the upper peripheral part of the grid to be translated into rotation of the top cap, so that the actual expansion of the top of the grid is held substantially equal to the thermal expansion of the lower portion of the grid.

Also in accordance with the present invention, the getter assembly for the tube is located within the tubular cathode supporting structure. A plurality of individual getter elements are secured between two spaced-apart discs having terminals arranged so that the getter elements may be fired independently of the voltages applied to other electrodes of the tube.

Referring to the accompanying drawings, in which corresponding reference numerals refer to corresponding parts in each of the figures:

Fig. 1 is a side elevation view, in section, of a power electron tube embodying the present invention;

Fig. 2 is a side elevation view of the screen grid electrode of Fig. l; and

Fig. 3 is a top plan view of the grid of Fig. 2.

Referring to Fig. 1, an air cooled electron tube 10 has a cathode assembly 11 comprising a cathode 12 Which comprises a hollow ring-like metallic element or annulus 13 whose diameter is positioned perpendicularly with respect to the longitudinal axis of the tube; the electron emitting portion 14 of the cathode being the outer peripheral surface of the element. The ringlike element 13 is of U-shaped cross section with the open end of the U facing inwardly towards the longitudinal axis of the tube. The electron emissive portion 14 of the cathode 12 is along the base of the U. The top and bottom surfaces of the ring-like element 13 represent the arms of the U.

The emissive surface on area 14 of ring-like element 13 is prepared by sintering a thickness of metal powder onto the smooth surface of the ring-like element 14 and then applying an emissive coating to the roughened sintered surface. The emissive coating adheres more readily to the roughened sintered surface than it would to a smooth metallic surface and has less tendency to peel or flake off during operation of the tube.

A toroidally wound cathode heater winding 15 whose wires are coated with a refractory insulating material is positioned in the recessed portion of the hollow ringlike cathode element 13. Since the heater Wires are in sulated, no supporting structure is needed for the heater coil 15. One of the heater wires is conductively secured to the ring-like element 13 and the other wire 32 is brought out to a conductive rod 16 which will be described later.

A cup-shaped metallic heat shield 17 which has a flanged lip 18 along the rim of the cup is conductively secured along the flanged lip to the top of the ring-like element 13, the cup portion extending downwardly into the tubular cathode support structure 19a, 19b of the cathode assembly 11.

The heat shield 17 effectively isolates the grid input from the alternating current fields set up by the heater winging 15. It also provides a surface for the reflection of heat back towards the emitting area 14 of the cathode 12. The bottom of the heat shield 17 has small apertures 20 which facilitate evacuation of the inside of the cathode assembly 11 during tube processing.

The upper part 19a of the tubular supporting structure for the cathode 12 is of smaller diameter than the cathode 12 in order to minimize capacitance between the cathode assembly 11 and the control grid structure 21. As illustrated, the tubular cathode supporting structure comprises an upper portion 19a which includes a thin walled heat isolating section 22, and a lower portion 19b of larger diameter. The cathode supporting structure 19a, 19b is, for the sake of convenience in manufacturing, made in two parts which are then welded or brazed together to form an integral structure. Alternatively, the cathode supporting structure could be made as a one piece structure of substantially the same diameter as the upper portion 19:: illustrated in Fig. 1. This alternative construction would require slight modification of the tube envelope due to the smaller diameter of the lower end of the cathode support.

The lower end 23 of the cathode supporting structure 1% is closed by a re-entrant apertured metallic member 24 which is a part of the tube envelope. The central cathode lead and terminal 25 and a lead and terminal 26 for the getter 38 pass through the aperture 27 of the re-entrant metallic member 24 and are insulated one from another by a sandwich type heremetic glass-to-metal seal 28.

The central cathode terminal 25 comprises an elongated hollow metallic cylinder 29 which has a metallic disc 36 conductively secured adjacent the end of the cylinder 29 which extends inside the tube envelope and has another disc-like flange 31 along the portion of the cylinder 29 which is outside the tube.

The getter terminal 26 comprises a hollow cylinder 32 of greater diameter than and coaxial with the central cathode terminal 25. A metallic disc 33 similar to the disc adjacent the end of the central cathode terminal 25 is conductively secured to the end of the hollow cylinder 32 of the getter terminal 26 which is inside the tube envelope. The end of the getter terminal 26 which is outside the tube has a flanged portion 34 of larger diameter than the second mentioned disc-like flange 31 of the central cathode terminal. In order to hermetically seal the central cathode terminal 25 and the getter terminal 26 to the re-eutrant metallic member 24, glass rings 35, 36 are placed between the central cathode terminal 25 and getter terminal 26 and between getter terminal 26 and the re-entrant metallic member 24, respectively. The seal is then made by applying heat and pressure between the members to be sealed.

The getter strips or capsules 37 are conductively secured between the discs 30, 33 on the cathode terminal 25 and the getter terminal 26 inside the tube envelope. Use of a separate getter terminal 26 in conjunction with the central cathode terminal 25 permits the getter 38 to be fired or activated at any time, regardless of the potentials applied to the other electrodes of the tube. Because the getter 38 is located within the tubular cathode support structure 19a, 1%, the getter flash as well as the getter structure is isolated from the radio frequency portions of the tube. The sandwich seal 28 is protected from the getter flash by the disc 33 on the end of the getter terminal 26.

The conductive rod 16, previously mentioned, is sealed to the central cathode terminal 25 and serves as one lead and terminal for the heater winding 15. Since the other end of the conductive rod 16 is terminated in an insulated bushing 39 on the heat shield 17, the rod 16 is provided with an expansion loop 40 to prevent strain on the sandwich seal 28 due to thermal expansion of the conductive rod 16 and the heat shield 17.

Further protection of the sandwich seal 28 is provided by the apertured re-entrant metallic member 24 which serves to isolate the sandwich type seal 28 from the strain which occurs when the rim 41 of the re-entrant envelope member 24 is subsequently welded to extension of the outer cathode terminal member 42.

The control grid 43, which is cup shaped, is telescoped over the cathode 12, the grid wires being closely spaced to the emitting surface 14 of the cathode 12. As in the case of the cathode support, 19a, 1%, the control grid 43 and its tubular support are, for the sake of convenience, made in two sections 44a, 44b. The upper section 44a includes the active portion of the control grid 43 (grid wires 45 and top cap structure 47 for interelectrode shielding) and a flanged base 46. A detailed description of the grid wires 45 and top cap structure 47 will be given later in connection with the screen grid structure 48, which is illustrated in Figs. 2 and 3. The grid wires and top caps of the control grid 43 and screen grid structure 48 are similarly constructed.

The upper section 44a of the grid structure 21 may be made by the method which is disclosed and claimed in U. 5. Patent No. 2,565,623 issued August 28, 1951, to W. N. Parker, and assigned to the same assignee as the instant case.

The lower section 44b of the control grid support comprises a tubular metallic body of substantially the same diameter as the flanged base 46 of the upper section of the control grid structure 44a. The lower end of the lower section 44!) has an outwardly extending skirt 49 which is welded, brazed, or otherwise conductively secured to control grid ring terminal 50 which is a part of the tube envelope. The base 46 and lower supporting section 44b of the control grid supporting structure provide a large area path for rapid conduction of heat from the active or grid wire portion of the control grid 43. The large spacing between control grid support 44a, 44b and cathode support 19a, 19b minimizes input capacity of the tube 19. The lower supporting section 44b is usually assembled with other parts of the tube before the upper supporting section 44a and control grid 43 are conductively secured to supporting section 44b.

The inwardly extending shelf 73 at the top of supporting section 4% allows the control grid 43 to be properly centered with the cathode 12 before sections 44a and 44b are secured together, as by brazing, for example. The bell-shaped screen grid structure 48 is telescoped over the control grid structure 21, with its base portion 51 conductively secured to a shelf-like annulus 52 which is in turn supported by an oifset metallic portion 53 of the tube envelope which is secured, by welding, for example, adjacent one of its ends to the screen grid ring terminal and envelope portion 54. The shelf-like annulus 52 permits proper centering of the screen grid structure 48 with respect to control grid 43 before the screen grid base 51 is secured to the shelf 52. The flared skirt of the screen grid base 51, provides wide spacing between screen grid and control grid support structures 51, 44a, which reduces interelectrode capacities between the two electrodes.

Referring to Figs. 2 and 3, the screen grid support 48 has a base 51 of larger diameter than the screen grid 55. The change in diameter between the screen grid 55 and the base 51 is accomplished by means of an outwardly extending tapered portion 56. The grid wires 57 of the screen grid 55 are positioned vertically (substantially parallel with the longitudinal axis of the tube) and spaced substantially equally distant from each other around the periphery of the screen grid structure 48. The top cap 58 provides interelectrode shielding, and is maintained in position by a plurality of similarly positioned elements or arms 59 which extend from a peripheral ring 60 which is interposed between the grid wires 57 and the elements or arms 59. As may be seen from Fig. 3, each of these arms 59 is positioned at an acute angle to a radius of the top cap 58.

Because the screen grid base 51 is of large cross section in order to provide a rigid mechanical support for the screen grid structure 48 and also to conduct heat from the screen grid 55 it has been found that the top portion of the grid wires 57, ring 60 and the top cap 58 are at a considerably higher temperature than the grid base 51 during operation of the tube 10.

Therefore, in view of the necessarily close spacing between the various tube electrodes, it is highly desirable that means be provided whereby both the upper and lower portions of the grid electrodes have the same change of diameter due to thermal expansion. Unequal expansion in different parts of an electrode tends to result in cumulative distortion and warping of the electrode. Equal expansion of top and bottom part of the screen grid 55 is accomplished in the grid of Figs. 2 and 3 by inclining the elements or arms 59 at an angle of, for example, 36 degrees to 45 degrees with respect to a radius of the top cap 58. The elements or arms 59 are, in the instant case, arranged substantially perpendicular to the longitudinal axis of the tube. With such an arrangement, thermal expansion or contraction causes the top cap 58 to be rotated. This rotation tends to pull in the outer peripheral ring 60 which would otherwise tend to expand more than the base portion 51 of the grid. By proper positioning of the arms or elements 59, diametrical thermal expansion or contraction of both the upper and lower parts of the active grid portion 57 can be held substantially constant.

The screen grid structure 48, like the upper part 44a of the control grid structure 21 can be made by the method of W. N. Parker previously referred to.

Referring again to Fig. 1, the active portions of cathode 12, control grid 43 and screen grid 55 extend into an inverted cup-shaped anode 61, which is part of the tube envelope and which is provided with a plurality of cooling fins 62. The exhaust tubulation 63 for the tube, which is located at the top of the anode, is covered by a protective cap 64 which is secured to the anode 61.

The upper part of the tube envelope includes the inverted cup-shaped anode 61, a metallic envelope portion 65 to which a flat annular anode terminal 66 is conductively secured, a ceramic ring 67, and the screen grid ring terminal 54. The lower end of the metallic envelope portion 65, and the upper end of the screen grid ring terminal 54 have outwardly extending tapered skirts which seat with and are hermetically sealed to complernentarily tapered surfaces 68a, 68b of the ceramic ring 67. The above mentioned parts of the envelope are usually assembled and sealed as a unit.

The lower portion of the envelope, which includes the outer cathode ring terminal 42, the control grid ring terminal 50, and offset or re-entrant metal envelope member 53, and two tapered ceramic rings 69, 70 interposed between the two terminals 42, 50 and the remnant member 53, to provide insulation therebetween, is likewise assembled and sealed as a unit in a manner similar to the upper part of the tube envelope.

The above mentioned two units of the envelope are hermetically sealed together by a weld or braze along the lower edge 71 of the screen grid ring terminal 54 and an edge of the reentrant envelope member 53. The reentrant envelope member 53 serves to provide an abrupt change in diameter of the tube envelope and to provide a long thermal path which protects the metal-to-ceramic seal (between 53, 70) at the other end of the re-entrant member 53 from the high temperatures encountered in making the welded or brazed seal between the two envelope units. The tube envelope is completed by inserting the cathode assembly 11 into the tube through the outer cathode terminal member 42.

Following insertion of the cathode assembly 11 into the tube, a weld or braze is made along the lower edge 72 of the extension of the cathode ring terminal 42 and the rim 41 of the apertured re-entrant member 24 through which the central cathode terminal 25 passes.

Referring to making of the metal-to-ceramic seals in more detail, the metal and ceramic bodies involved in the seal are provided with a 30 degree taper and a matching diameter. The metal body may, for example, be made of a base metal alloy comprising approximately 50 percent nickel and 50 percent iron. Such a metal alloy has been found to roughly match the thermal expansion of the ceramic used in making the ring seals. One ceramic material which has been successfully used in the tube envelope is fosterite.

Any suitable known method may be employed in making the metal-to-ceramic seals. For example, the sealing surface of one of the parts may be coated with a layer of glaze or glass prior to assembly of the parts in a furnace, as described in U. S. Patent No. 2,210,699 to W. E. Bahls. Alternatively, the ceramic part may be metallized and then soldered to the metal part, as described in U. S. Patent No. 2,163,410 to H. Pulfrich et al.

When an assembly of parts i needed, a firing jig (not shown) is stacked with the proper parts and sent through a furnace having a neutral (nitrogen) atmosphere at a temperature sufficiently high to effect an hermetic seal between the parts. While the assembly is in the furnace the parts are free to move in a vertical direction, which avoids warping.

Tapered ceramic-to-metal seals result in an improved envelope construction over that possible when conventional flame sealing is done. When flame sealing techniques are used, the quality of the seal and the positioning of the parts depends largely upon the skill of the technician doing the work. Likewise, when flame heating such a seal, an oxidizing atmosphere is present which oxidizes the metallic surfaces in the vicinity of the heat. Then, too, a flame sealing often results in warping of the materials because of the wide differences in temperatures in different areas of the materials. Also, when parts are flame sealed, chemical cleaning of the oxidized surfaces is imperative, and if the internal structure of the tube contains re-entrant portions of the type found necessary in the instant case, satisfactory chemical cleaning would be very difficult to accomplish. Unless good chemical cleaning is obtained however, contamination and poisoning of the cathode are highly probable. These difficulties are avoided in the tapered seal of the present invention because of the self-jigging of the seal, and the fact that it is fired in a neutral or non-oxidizing atmosphere so that no chemical cleaning is necessary.

In addition, the use of ceramic seals allows a much higher bake out temperature during tube processing than would be possible if metal-to-glass seals were used. This higher bake out temperature results in a harder vacuum. Also, since the seals can withstand higher temperatures without fracturing, tubes incorporating such seals can be safely operated at higher power levels than if metal-toglass seals were used. From an electrical standpoint, the specific resistance and dissipation factor of the ceramic have been found to be better than any glass available.

What is claimed is:

1. An electron tube having, within an envelope, a cathode-getter assembly, said assembly comprising a tubular cathode structure and cathode supporting structure, said cathode structure having at least one aperture therein, said cathode structure secured to said supporting structure adjacent to one end thereof, an envelope member closing the other end of said supporting structure, coaxial getter leadterrninal members disposed inside said supporting structure and extending through said envelope member, a pair of spaced plates one conductively secured to each of said lead-terminal members inside said supporting structure, and at least one greater element, an end portion of said getter element being secured to each of said plates.

2. An electron tube having, within an envelope, 2. cathode-getter assembly, said assembly comprising a tubular cathode and supporting structure, said cathode secured to said supporting structure adjacent one end thereof, an envelope member closing the other end of said supporting structure, coaxial getter lead-terminals inside said supporting structure and extending through said envelope member, a plate secured to each of said lead-terminal members inside said supporting structure, and at least one getter element, each end of said getter element being secured to one of said plates.

3. A getter assembly for an electron tube having an envelope, said assembly comprising inner and outer coaxial conductive members which extend through and are insulated from said tube envelope, said members being electrically insulated one from the other by an hermetic seal, said inner member extending beyond said outer member on both ends thereof, a pair of plate-like elements, one of said plate-like elements being conductively secured to each of said members, and at least one getter element conductively connected across said plates.

4. A grid-cathode sub-assembly for an electron tube having a multi-section envelope, said sub-assembly comprising: a cathode assembly including a tubular indirectly heated cathode, a tubular cathode support having a portion of smaller diameter than said cathode, said cathode being conductively secured to said smaller diameter portion of said tubular support, and a cathode terminalenvelope member conductively secured to said tubular support, said terminal envelope member having an outwardly flared skirt; a grid assembly comprising a tubular grid of openwork construction including a top cap, a first tubular grid support having an outwardly extending flange adjacent to one end, said grid being conductively secured to said first grid support adjacent the other end thereof, and a second tubular grid support of larger diameter than said first grid support, one end of said second grid support being conductively joined to said first grid support at the flanged end thereof, the other end of said second grid support being conductively secured to a grid terminal-envelope member which has an outwardly flared skirt, the flared skirt of said grid terminal-envelope member being adjacent to but spaced from the flared skirt of said cathode terminal-envelope member; and an insulating spacer having peripheral surfaces complementarily inclined with respect to said skirts positioned between said grid and cathode assemblies, said flared skirts of said grid and cathode assemblies each being seated with and hermetically sealed to an inclined surface of said insulating spacer.

5. A cathode assembly for an electron tube, comprising a hollow tubular support member, a hollow annulus of substantially U-shaped cross section positioned coaxial with and secured to said tubular support member, an outer surface of said annulus coated with an emissive material, a cup-shaped metallic heat shield, the side of said shield being positioned within the open central portion of said hollow annulus and secured thereto and closing the same, and a heater winding disposed within the enclosure formed by said hollow annulus and said shield, said winding having leads which are insulated one from another, an apertured member hermetically sealed to said support member adjacent to the end of said tubular support member which is remote from said hollow annulus, at least one of said leads having an end portion passing through said aperture and being hermetically sealed thereto, and the other end of said one lead being insulatingly connected to and supported by the base of said cup-shaped shield.

6. A cathode assembly for a power electron tube, comprising a hollow tubular support member having a thin walled portion adjacent one end thereof, a hollow annulus positioned coaxial with and secured to said thin walled portion, an outer surface of said annulus being coated with an electron emissive material, a cup-shaped metallic heat shield having an outwardly extending peripheral flange along the rim of said cup, the side of said cup being positioned within said hollow annulus, said flange being se cured to said hollow annulus remote from said thin walled portion, a conductive rod insulatingly secured to said cupshaped shield and disposed within said hollow tubular support member, a cathode heater disposed within said hollow annulus, said heater having a lead electrically connected to said rod and a lead electrically connected to said hollow annulus, an apertured metal member hermetically sealed to the end of said support member remote from said thin wall portion, a pair of coaxial lead-terminal members extending through the aperture of said apertured metal member and being insulatingly hermetically sealed thereto and from each other, said conductive rod being conductively secured to the inner lead-terminal memher, a plate secured to each of said coaxial lead terminals, and at least one getter element, an end portion of said getter element being secured to each of said plates.

7. A cathode assembly for a power electron tube, comprising a hollow tubular support member having a thin walled portion adjacent one end thereof, a hollow annulus of substantially U-shaped cross section and having a larger outer diameter than said support member positioned coaxial with and conductively secured to the end of said support member adjacent to said thin walled portion, an outer surface of said annulus being coated with an electron emissive material, a cup-shaped metallic heat shield having an outwardly extending peripheral flange along the rim of said cup, the side of said cup being positoned within the central opening of said hollow annulus, said flange being secured to said hollow annulus remote from said thin walled portion, a conductive rod insulatingly secured to said cup-shaped shield and extending within said tubular support member, said rod having an expansion offset, a multistrand toroidal heater disposed within said hollow annulus, said heater having a lead electrically connected to said rod and a lead electrically connected to said hollow annulus, an apertured metal member hermetically sealed to the end of said support member remote from said thin walled portion, said conductive rod extending through said aperture, and an insulating hermetic seal between said rod and the periphery of said aperture.

8. A cathode assembly for a power electron tube, comprising a hollow tubular support member having a thin walled portion adjacent one end thereof, a hollow annulus positioned coaxial with and secured to said thin walled portion, an outer surface of said annulus being coated with an electron emissive material, a cupshaped metallic heat shield having an outwardly extending peripheral flange along the rim of said cup, the side of said cup being positioned in the open center of said annulus, said flange being secured to said hollow annulus remote from said thin walled portion, a conductive rod insulatingly secured to said cup-shaped shield and extending within said tubular support member, a multistrand toroidal heater disposed within said hollow annulus, said heater having a lead electrically connected to said rod and a lead electrically connected to said hollow annulus, an apertured metal member hermetically sealed to the end of said support member remote from said thin walled portion, said conductive rod extending through said aperture, and an insulating hermetic seal between said rod and the periphery of said aperture.

9. A cathode assembly for a power electron tube, comprising a hollow tubular support member having a thin walled portion adjacent one end thereof, a hollow annulus of substantially U-shapcd cross section and having a larger outer diameter than said support member positioned coaxial with and secured to the end of said support member adjacent to said thin walled portion, an outer surface of said annulus being coated with an electron emissive material, a cup-shaped metallic heat shield having an outwardly extending peripheral flange along the rim of said cup and having at least one aperture, the side of said cup being positioned within the central opening of said hollow annulus, said flange being secured to said hollow annulus remote from said thin walled portion, a

conductive rod insulatingly secured to said cup-shaped shield and disposed within said hollow tubular support member, said rod having an expansion offset, a cathode heater disposed within said hollow annulus, said heater having a lead electrically connected to said rod and a lead electrically connected to said support member, an apertured metal member hermetically sealed to said support member adjacent the end thereof which is remote from said thin walled portion, a pair of coaxial lead-terminal members extending through the aperture of said apertured metal member and being insulatingly hermetically sealed thereto and from each other, said conductive rod being conductively secured to the inner lead-terminal member, a plate secured to each of said coaxial lead terminals, and at least one getter element, an end portion of said getter element being secured to each of said plates.

10. A cathode assembly for an electron tube, comprising a hollow tubular support member, an inwardly-open hollow annulus of substantially U-shaped cross section positioned coaxial with and secured to said tubular sup port member adjacent to an end thereof, an outer surface of said annulus being coated with an emissive material, a cup-shaped metallic heat shield, the side of said shield being secured to said hollow annulus and extending downwardly to substantially close the open side of said annulus, and a heater winding disposed within said hollow annulus, said winding having leads, an aperturcd metallic member hermetically sealed adjacent an end of said tubular support member which is remote from said hollow annulus, at least one of said leads passing through said aperture and being insulatingly hermetically sealed there to.

11. A cathode assembly for an electron tube, comprising a hollow tubular support member, an inwardly-open hollow annulus of substantially U-shaped cross section and having a larger outer diameter than said support member positioned coaxial with and secured to said tubular support member adjacent to an end thereof, an outer surface of said annulus being coated with an emissive material, a cup-shaped metallic heat shield, the side of said shield being secured to said hollow annulus and extending downwardly to substantially close to the open side of said annulus, and a heater Winding disposed within said hollow annulus, said winding having leads, an apertured metallic member hermetically sealed adjacent an cud of said tubular support member which is remote from said hollow annulus, at least one of said leads passing through said aperture and being insulatingly hermetically sealed thereto.

References Cited in the tile of this patent UNITED STATES PATENTS 

